docs: def head(self, x, y, layers=None, aq=None):
          """Head at `x`, `y`
          
          Returns
          -------
          
          h : array length `naq` or `len(layers)`
              head in all `layers` (if not `None`), or all layers of aquifer (otherwise)
          """
          
          if aq is None: aq = self.aq.find_aquifer_data(x, y)
          rv = self.potential(x, y, aq) / aq.T
          if layers is None:
              return rv
          else:
              return rv[layers]